At-home genetic tests are all the rage. But can they really calculate your risk of developing life-threatening diseases? Before buying one, read this Prevention special report.

Anna Peterson is only 27, but she's already watched her mother and her aunt develop breast cancer. She also saw her grandmother's eyesight fail from macular degeneration. So Peterson, a graduate student in Ottawa, Ontario, took her health care into her own hands and did what millions of others of all ages are doing: She opted for at-home DNA testing for genetic diseases.

When the results from the $985 test from deCODEme arrived in her e-mail, Peterson felt relief learning that she didn't have an elevated genetic risk for macular degeneration. Yet the test did show a slightly elevated risk for the more common forms of breast cancer. The results, Peterson says, empowered her to make healthier choices. Together with her physician, she'll use that information to advocate for earlier screening for breast cancer--and possibly start getting mammograms at age 30. "Prevention starts with knowing the odds," says Peterson. "I now have the opportunity to make lifestyle changes in my 20s, rather than in my 60s."

Welcome to the brave new world of genetic analysis. Once the exclusive domain of doctors and genetic counselors, DNA testing is now a do-it-yourself proposition, with several dozen companies marketing tests directly to consumers, claiming that they will allow you to understand your genetic profile. The process is surprisingly simple: Buy a test online, swab the inside of your cheek or spit into a test tube to collect a DNA testing sample, and then mail it to the company. In return, you'll receive personalized medical information that purportedly allows you to combat genetic diseases by making informed choices about your health. Bolstering that promise is new research that shows you can actually turn off genes that promote certain diseases by improving your diet and better managing stress.

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Genetic Testing Goes Mainstream

People clearly approve of DNA testing. In a recent Prevention.com poll, 87% of respondents said they'd want to know which genetic diseases they're at high risk of developing. Moreover, 54% said they'd be likely to have a genetic test even if there was no known treatment or way to prevent the disease.

Plenty of companies are eager to meet this demand, selling at-home tests that range in cost from hundreds to thousands of dollars. Some offer tests that have long been available through doctors and genetic counselors--for instance, those that check for BRCA 1 or 2, the genes linked to a small percentage of inherited cases of breast cancer. Newer versions look at your SNPs (pronounced snips, short for "single nucleotide polymorphisms"), the slight variations within DNA that can account for differences in appearance and how we develop genetic diseases. The companies don't predict that you're going to get, say, cancer or macular degeneration. Rather, you get back a report showing the risk you run, compared with the average person. One of the newest entrants into the at-home arena, Navigenics, recently launched its $2,500 Health Compass test, which looks for markers associated with 23 common conditions--including diabetes, prostate cancer, and Alzheimer's disease--that are "actionable," or able to be prevented or detected early. For an additional $250 per year, subscribers receive personalized updates when relevant genetic research--for instance, the discovery of new SNPs--changes their health outlook.

Health-conscious consumers are clearly enamored with these high-tech tests--they're expected to spend an estimated $6 billion to have their DNA decoded over the next 5 years. But while the business of do-it-yourself DNA testing is booming, experts say this new frontier of medicine isn't ready for prime time. They worry that the field is insufficiently regulated, not all of the tests are reliable, and the information garnered is incomplete and possibly misleading. "Some tests lack adequate scientific evidence to support their use, and the lack of regulation means there's no way for consumers to separate the good from the bad," says Gail Javitt, law and public policy director for the Genetics and Public Policy Center at Johns Hopkins University.

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The Risks of Peering into Your Medical Future

Despite an explosion in the discovery of SNPs, most experts say it's too early to make strong links between these tiny genetic variants and the development of genetic diseases. That's because the role played by a single mutation is believed to be modest. Rather, it's the interaction between multiple SNPs and factors like diet, exercise, and weight that predispose you to disease--indeed, studies show that lifestyle accounts for about 70% of our susceptibility to health problems such as diabetes, heart disease, and some types of cancer. As a result, "these tests can't accurately predict the risk of developing these complex diseases at the moment," says David Hunter, MD, ScD, professor of epidemiology and nutrition at the Harvard School of Public Health. Likewise, some tests that claim to gauge your risk of diseases such as depression examine single genes that explain only a small part of the predisposition to those illnesses.

Critics also note that DNA testing results can be tough to interpret, making it difficult to know when and if you should take action. A Navigenics test told Robert C. Green, MD, PhD, MPH, a professor of neurology, genetics, and epidemiology at Boston University Schools of Medicine and Public Health, that he had a 20% above-average risk of developing multiple sclerosis. But the average risk is 0.3%, and his was just 0.5%. These are both very low risks (3 out of every 1,000 versus 5 out of every 1,000, respectively), yet the results were highlighted in orange, indicating an elevated risk. Green, a geneticist, understood his real risk, but the average person might not, causing needless worry.

And consider the flip side: that someone who tests negative for a gene or is told she's at low risk for developing a dreaded disease becomes less motivated to lead a healthy lifestyle. David Katz, MD, MPH, director of the Prevention Research Center at Yale University School of Medicine, points to a study on premenopausal women given a range of tests to determine their risks of developing heart disease. Half saw scans of their coronary arteries, which were surprisingly healthy. But despite other risks revealed from triglyceride and blood glucose levels, the group that saw its healthy scans did less to follow recommendations to prevent heart disease. "We don't want a single test talking people out of taking care of themselves, and this study suggests that can happen," says Katz.

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Test results: helpful or confusing?

Proponents of mail-order genetic testing claim it has just the opposite effect and instead helps people gain new insights that sometimes dramatically boost their health. David Agus, MD, an oncologist and cancer researcher at the University of Southern California, started Navigenics with the hope that people would get tested, discuss results with their physicians, and use the information to seek an earlier diagnosis or delay the onset--or prevent--certain conditions altogether. That's precisely what happened in the case of Mari Baker, the company's CEO. When her results revealed a 5-times-greater risk of celiac disease, follow-up testing ordered by her physician confirmed she has the digestive disease, which is caused by an intolerance to gluten, a protein in wheat, rye, and barley. The diagnosis explained the GI problems Baker, 44, suffered over the years. She immediately changed her diet and stocked her kitchen with gluten-free pasta, bread, and beer. "Feeling a little bit better every day for the next 20 years is pretty important," she says.

Unfortunately, test results aren't always so clear-cut and accurate. Mike Spear, the communications director for a genomic research nonprofit in Alberta, took tests from deCODEme and 23andMe and received some conflicting findings. For instance, the deCODEme test showed a higher-than-average risk for MS, but the 23andMe test said his risk was no higher than that of the average person. The test from deCODEme also had a big check mark next to "male pattern baldness," while the 23andMe test said he was on par with the rest of the population. (Spear, who's 55, still has plenty of hair.) Most surprisingly, both tests told Spear that his risk of asthma was no different from the average person's--even though he already suffers from the disease. His impression: "I'd be careful about basing your life around genetic test results."

Although new genetic research emerges every day, scientists haven't discovered all the genetic variants--or all the SNPs--for common diseases. Green, 54, took two genetic tests, one from 23andMe and the other from Navigenics, and both told him that he had an average risk of cardiac disease. Yet he had undergone triple bypass heart surgery the previous year. As a runner who is not overweight, doesn't smoke, and has low cholesterol, Green suspects there's a genetic cause for his tendency toward blocked arteries--and he chalks up the discrepancy in his test results to the fact that more research is needed to fully understand the genetic causes of cardiac disease.

Another reason to think twice about these tests: The industry is a virtual free-for-all. No single government agency watches the labs performing the tests to ensure that the science behind the tests is even real. Currently, the FDA reviews most other home-use medical tests for safety and effectiveness, but at-home genetic tests don't fall under the agency's territory. Pending legislation would require makers of direct-to-consumer genetic tests to prove that their tests are accurate and properly performed. But right now, "the public's best approach is buyer beware," says Kathy Hudson, PhD, founder and director of the Genetics and Public Policy Center at Johns Hopkins.

Despite these warnings, experts predict that people will be very tempted by the chance to peek into the genetic crystal ball. If you're one of them, heed Katz's reminder: "DNA isn't destiny." And be aware that when it comes to the lifestyle choices that are the greatest predictors of health, your future is in your hands.

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Breakthrough: Should you customize your drugs to your DNA?

Here's a shocker: Due to differences in DNA, up to 60% of the most common drugs are associated with adverse reactions. This includes medication used to treat common conditions like hypertension, heart failure, depression, high cholesterol, and asthma.

Hence the hope being pinned on "pharmacogenetics," a field of medicine that promises to improve health care by allowing doctors to customize medical treatment to suit a person's unique genetic signature. Though experts predict that it could be decades before personalized medicine becomes the norm, research is moving ahead: Last fall, for instance, researchers at Duke University reported that people with a specific genetic variant saw less reduction in LDL, or "bad" cholesterol, when taking statins.

But for some drugs, the future is now. A genetic test recently approved by the FDA should help doctors determine the optimal dose of warfarin (sold as Coumadin), a blood thinner used by 1 million Americans. Determining the right dose is crucial: Too much may result in an increased risk of excessive bleeding, while too little may cause a potentially fatal blood clot. By one estimate, using DNA analysis to prescribe warfarin would prevent about 17,000 strokes and 85,000 serious bleeding incidents.

A small but growing number of doctors and hospitals are also using genetic testing to tailor treatment for these medicines:

Tamoxifen DNA testing identifies the 8% of women with genetic variants that keep them from metabolizing the breast cancer drug, rendering it ineffective.

Painkillers like codeine Up to 8% of whites and 2% of Asians and African Americans are poor metabolizers of these drugs and won't get relief from them; for the 1% of "ultrarapid metabolizers," risks include respiratory problems.

Antidepressants and antipsychotics Some of these drugs are metabolized by the CYP2D6 and CYP2C19 genes. In 2005, the FDA approved a test that looks for these gene variations, and now companies sell consumer versions. But experts advise against using the at-home tests without having your doctor interpret the results, notes Julie Johnson, PharmD, professor of pharmacy and medicine at the University of Florida. The reason: These genes are involved in the metabolization of 25% of all prescription drugs, including several where they're very important. If you misinterpret the results of an at-home test (and mistakenly think you don't have the gene), you might avoid taking one or more drugs you really need.

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Which genetic tests make the grade?

Decent

Tests for diseases with a proven link to single genes. Unlike SNPs tests, which look at gene variations that may put you at higher risk for diseases, some tests screen for specific gene mutations that have been scientifically proven to either definitively cause a disease or greatly increase your chance of developing one. Some disorders that can be screened for this way include breast cancer, colon cancer, Huntington's (a neurological disorder), and hemochromatosis (when the body stores too much iron). When people concerned about these diseases are trying to protect their privacy, some opt to use at-home versions of tests so the results won't be part of medical records. Companies that offer this type of testing include DNA Direct, Kimball Genetics, and Myriad Genetics.

Consider with caution SNPs tests from companies that use a laboratory certified by CLIA, which sets standards for US clinical laboratories. This accreditation ensures that the company uses laboratories that adhere to standards and guidelines for clinical testing. Companies using CLIA-certified labs include DNA Direct, deCODEme, and Navigenics.

Companies that offer genetic counseling. Some genetic tests deal with statistical risk that can be tough to understand and needs to be considered with your family history, so be sure a knowledgeable health professional interprets your test to avoid needless anxiety and any rash medical decisions. Some companies include free online or telephone sessions with certified genetic counselors and send detailed reports to help explain what test results really mean.

Avoid

"Nutrigenomic" tests that promise to identify your risk of certain diseases and then sell you expensive vitamin regimes that are supposedly based on your genetic profile and help you prevent disease. The General Accounting Office, the investigative arm of Congress, looked at a number of these companies and found that some of these recommended supplements cost $1,200 a year and were actually similar to supplements found in stores for $35. In some cases, the vitamins exceeded recommended daily allowances, making them potentially harmful, and regardless of the different DNA sample, the "personalized" supplements sent were all the same. GAO investigators couldn't verify any of the scientific claims made by manufacturers of these tests.

Tests from companies without strict privacy policies. Despite last year's passage of the Genetic Information Nondiscrimination Act, which prohibits your insurance company and employer from using your genetic information against you, no laws yet exist to limit what genetic testing companies can and cannot do with a person's genetic information or DNA sample. Be sure that a company securely stores your DNA sample and will not sell it to be used in a research study without your permission. If the company doesn't provide this information on its Web site, ask for it directly. "Using your information without your consent is unethical," says Caroline Lieber, director of the graduate program in human genetics at Sarah Lawrence College.

Tests that don't provide documented scientific evidence validating their claims. You should be fully informed about what a test can and cannot say about your health. Companies should make scientific references available on their Web sites to document the data used for the tests. If they don't offer it, ask for it.

Even if you have one of the 18 genes linked to diabetes, it's not as if it's hardwired into your DNA that you'll get the disease on your 40th birthday--or even at all